One of the things I find most odd about some people, is the hard and fast view about whether something 'exists' or not, if it's outside of their own experience. Especially in the field of human relationships, where things are rarely neatly packaged into a box and not everyone behaves the same way as their counterparts, has the same relationships as everyone else or thinks and feels like everyone else either.
What's even odder is when they continue to say 'I don't believe it exists' when lots of people are telling them that yes it does exist, it happened to them, or someone close to them.
In my time on mumsnet, I've read loads of threads from people who say they are in good marriages but are having affairs. I've met people in real-life who've said the same. This leads me to conclude that if so many people are saying something, there must be something in it and they aren't lying.
But in a more general sense- and this only applies to mumsnet I'm afraid, I've seen posters insist that 'all men wank' or 'all men use porn' despite known regular male posters coming on to say that they don't. I was on a thread a few months ago where several posters were talking about working for firms that had policies in place forbidding partner relationships in the management hierarchy, only for one poster to say that because neither her nor her husband had heard of it, this was either rare or non-existent.
I do wonder why this sort of bull-nosed denial goes on.
Getting back on topic, I find this myopia about marriages having extraordinary deterrent powers as absurd as if someone said no-one becomes an alcoholic or drug dependent if they are in a good marriage. Or even, no-one would ever be violent or cruel to a partner if the relationship was good from their side.
Not having anything particularly invested in such hard and fast views, my take on this is that affairs happen for all sorts of reasons. I've known people have affairs as an exit from a lousy marriage and/or emotional/physical abuse, I've known people who've had affairs and had good faithful subsequent relationships with the OW/OM and I've known people have 'bit of fun' affairs like the one the OP's describing, despite being in a good marriage and not wanting to lose it. I'd never be so presumptuous to say something doesn't exist or can't happen just because it's outside of my own relatively small reach in the world though.
And if despite hearing from people who told me that it did and I was still insisting it didn't, I'd have to question why I was having such difficulty with other people's reality. There's usually a reason for that after all.